A further object is to provide an engine in which the effective compression ratio remains relatively high and is not reduced at low throttle settings.
More specifically, the invention provides an ignition engine comprising a cylinder head, a cylinder and a piston reciprocable therein, first inlet means for admitting a first gas which contains fuel into a central region of the cylinder, and second inlet means for admitting a second gas which does not contain fuel or contains significantly less fuel than the first gas, said second gas being confined generally adjacent to the cylinder walls, and an ignition device located generally centrally of the cylinder head.
In practice, the first gas will normally comprise an air-fuel mixture and the second gas will comprise air.
In one arrangement the first and second inlet means may comprise first and second ducts, the first duct being arranged to admit the first gas generally axially of the cylinder, the second duct being arranged to admit the second gas with a tangential component of velocity relative to the cylinder.
The two ducts may be connected to a common inlet port formed in the cylinder head of the engine, said port being opened or closed by a valve. Alternatively, the ducts may be connected to respective inlet ports in the cylinder head each being opened or closed by respective valves.
Valves are preferably provided in the ducts for controlling flow therethrough, said valves being arranged to operate in complementary manner so that as the volume of flow of the first gas increases the volume of flow of the second decreases and vice versa. The arrangement is such that at low throttle settings when small charges of air-fuel mixture are admitted to the cylinder, a significantly greater proportion of air is admitted to the cylinder through the second duct. On the other hand, at full throttle, the valves are arranged to admit a full charge of air fuel to the cylinder and to substantially block flow of air to the cylinder via the second duct.
In an alternative arrangement, the first duct is connected to an inlet port formed in the cylinder head and the second inlet means comprises ports formed through the cylinder wall, which ports are opened by the piston as it sweeps past the cylinder wall near the bottom of its stroke. In this arrangement, at low throttle settings a considerable vacuum will be present in the cylinder at or near bottom dead centre so that when the ports are opened air will enter the cylinder with considerable velocity. By arranging for the ports to be tangential and/or the piston top to be formed appropriately the inrushing air swirls about the inner surface of the cylinder walls. The air having a much higher velocity than the fuel-rich charge tends to remain adjacent to the cylinder wall and not to mix significantly with the fuel-rich charge. Provided the air has sufficient tangential velocity, separation will be maintained during the compression stroke of the piston whereby the air-fuel charge, which is of approximately stoichiometric proportions, is not diluted with the admitted air and hence reliable ignition can still be obtained.
The presence of air swirling about the interior of the cylinder wall also has the beneficial effect of reducing heat losses through the cylinder wall thereby increasing efficiency of the engine and decreases the problem of cooling of the engine.
The invention also provides a head for use on an internal combustion engine said head having a lower surface for mounting on an engine block a recess formed into said surface to serve as a combustion chamber in use of the head, an air-fuel inlet duct and an exhaust duct opening into the recess characterised by the provision of an air duct which opens into the recess and is directed tangentially relative thereto.